1. Field of the Invention
The present invention relates generally to ink jet printers for forming ink images on recording media and more particularly to a transfer type ink jet printer for transferring an ink image to a recording medium by pressing a transfer medium against the recording medium face to face after forming the ink image on the transfer medium.
2. Related Art
A printer of the sort disclosed by, for instance, U.S. Pat. No. 4,538,156 is known as a conventional transfer type ink jet printer.
This printer is, as shown in FIG. 25, designed to transfer an ink image by pressing recording paper 12 against the ink image formed by a recording head 11 on the surface of a cylindrical transfer medium 10. The residual ink left untransferred is removed by a cleaner 13.
The conventional transfer type ink jet printer has put it in question that the quality of the image thus transferred is not always good.
In the first place, the reason for this is that when the ink image formed of liquid ink makes contact with the recording paper, the liquid ink soaks along the fibers, thus allowing whiskers to grow on the periphery of the ink image.
In the second place, the reason is that since the image transfer is effected by the contact between the transfer medium and, the recording paper, an recessed portion of the recording paper will not contact the transfer medium if the smoothness of the recording paper is poor, and there appears a so-called "whitening" phenomenon in which an image to be transferred is not transferred.
In view of these problems, the present applicants propose in Japanese Patent Laid-Open No. 92849/1987 an apparatus for transferring an ink image to recording paper by ejecting ink droplets to a transfer medium once and evaporating a solvent composition in the ink droplets in order to press the concentrated ink image against the recording paper. This apparatus is, as shown in FIG. 26, designed to transfer an image by pressing a transfer medium 22 to which ink has been ejected from a recording head 21 against a recording medium 25 supported with a roller 24; in this case, a heater 23 is used to accelerate the evaporation of a solvent composition in the ink. The transfer medium 22 after the transfer operation is cleaned with a brush and dried by a blower 27 to keep the surface condition constant.
With this apparatus, the ink image is prevented from deforming while it is being transferred or from soaking into the recording medium 25 since the concentrated ink image is transferred to the recording medium 25. Therefore, clear image formation can be made possible by solving the problems characteristic of transfer type ink jet printers.
U.S. Pat. No. 5,099,256 also proposes an apparatus for transferring ink to recording paper by ejecting ink droplets to a transfer medium once and evaporating the solvent composition in the ink droplets in order to press the concentrated ink against the recording paper. This apparatus is, as shown in FIG. 27, designed to transfer an image by pressing a transfer medium 32 onto which ink has been ejected from a recording head 31 against a recording medium 35 supported with a roller 34; in this case, a heater is used to accelerate the evaporation of a solvent composition in the ink.
With this apparatus, the ink image is prevented from deforming while it is being transferred or from soaking into the recording medium 35 since the concentrated ink is transferred to the recording medium 35. Therefore, clear image formation can be made possible by solving the problems characteristic of transfer type ink jet printers.
However, these apparatus still have the following problems to solve from the viewpoint of the use of such a heater for thickening ink.
In the first place, it will take time during the process of evaporating the solvent composition if the heater temperature is low, whereas though the solvent composition may be evaporated for a short time at high temperatures, power consumption tends to increase as the heater temperature is raised. Further, it will still take time to raise the set temperature. In other words, the problem is that a demand for high-speed printing has not been satisfied.
In the second place, the inside of the apparatus may be contaminated since the solvent steam is discharged into the apparatus as the solvent evaporates.
In the third place, there arises a problem in that the transfer pressure may increase. More specifically, the time required from the ejection of ink up to the transfer of an image tends to vary with the spot when one nozzle head or a multiple nozzle head is used for scanning even though it is attempted to specify the concentrated ink condition by setting the quantity of heat at a proper value. This means there occurs a disparity in the concentrated ink condition. In this case, the relationship between the concentrated ink image condition and the transfer pressure is such that the greater the concentration of the ink used to form the imager the greater the transfer pressure becomes. It is therefore necessary to set the pressure value in conformity with the maximum concentration of the ink used for forming the image and this tends to require an excessive pressure. Further, an excessive transfer pressure may also be required as the concentration of the ink used for forming the image varies with the printing pattern of the ink image, that is, the amount of ink per unit area.